At least 19 people suffered injuries, said Fire Department of New York Deputy Chief James Leonard. Four of those injuries were considered serious because the people were complaining of chest pain. The rest of the injuries were minor.

The middle six cars of the Brooklyn-bound F train went off the express tracks south of 65th Street in the Woodside neighborhood around 10:30 a.m., Metropolitan Transportation Authority officials said. The first and last cars stayed on the tracks.

A person familiar with the MTA’s investigation said the cause initially appears to be related to the track and not related to the speed of the train.

The train’s operator and conductor will undergo routine drug and alcohol tests, said MTA President Thomas Prendergast.

Riders said the train rocked sharply before it derailed. Rescue crews cut power to the tunnel—leaving only emergency lights on and stopping air circulation—to turn off the electrified third rail so that passengers could be evacuated, Mr. Leonard said.

It was “hot for people down there” Mr. Leonard said. Firefighters and emergency medical service workers were placed into every car, he said, to assure people “you are safe, we will get you out of here.”

The passengers are being led through the tunnel and to emergency exits, he said. The entire process took about an hour.

Riders described a thick smoke underground, but Mr. Leonard said it was a cloud of dust.

Caisha Jean Philippe, 21, of Hempstead, Nassau County, said, “I thought it was an earthquake.”

“I thought it was driving too fast then the train just” stopped, she added.

Victims come out from the train derailment in Queens. See more photos in our slideshow.

John Taggart for The Wall Street

Gabrielle Heslop, 25, said she was heading to 34th Street.

“It was like really scary, we felt like it hit something. We thought we crashed into a train, or we thought the train was on fire.”

Anna Rodriguez sobbed and shook after her rescue. She was visiting family in Jamaica and was on her way back home to Manhattan.

“All of us started shaking from side to side,” she said. “I thought we were going to die there, I thought it was going to turn over.”

She said passengers were gripping the seats.

There were two other trains behind the derailed F that had to be brought back to the 71st Ave. station, the MTA spokeswoman said.

An E train was stranded after the derailment because subway crews turned off power to the tracks. Once the passengers are removed from the derailed F train, the stranded E train will be brought back to a nearby station.

The MTA spokeswoman said she didn’t know how many train cars came off the tracks or what might have caused the derailment.

Sarah Johnson, 17, was riding with her mother from Jamaica to Hunter College, where she was recently accepted as a student.

“Out of nowhere the train started to shake,” she said. “Everyone started screaming. I’m just happy to be alive.”

“Everyone was having trouble breathing,” she added. “I said to myself, it’s my time to die. As it’s tilting, I said to myself this is the end of me and my mother.”

She said she was stuck for about 20 minutes before she was led out of the tunnel.